1. Field of the Invention
The present invention relates to a reversible thermosensitive recording medium, more particularly to a reversible thermosensitive recording medium comprising a reversible thermosensitive recording layer of which transparency or color tone is reversibly changeable depending upon the temperature thereof, thereby recording information therein and erasing recorded information therefrom repeatedly as desired. The reversible thermosensitive recording may be used in information recording devices in any form, for instance, in the form of a card, a disk, a label, or a disk cartridge. The present invention also relates to a method of producing the above reversible thermosensitive recording medium. The present invention also relates to a method of image formation and erasure, using the reversible thermosensitive recording medium. The present invention furthermore relates to an apparatus for performing the above method of image formation and erasure, using the reversible thermosensitive recording medium.
2. Discussion of Background
Recently attention has been paid to a reversible thermosensitive recording material capable of temporarily recording images therein and erasing the same therefrom when such images become unnecessary. For example, as disclosed in Japanese Laid-Open Patent Application 55-154198, there are conventionally known reversible thermosensitive recording materials in which an organic low-molecular-weight material such as a higher fatty acid is dispersed in a matrix resin such as a vinyl chloride-vinyl acetate copolymer.
However, such a conventional reversible thermosensitive recording material has a shortcoming that a temperature range in which the recording material exhibits light transmission or transparency characteristics or is in a transparent state (hereinafter referred to as the transparentizing temperature width) is as narrow as 2 to 4xc2x0 C., so that it is difficult to control the temperature for performing such image formation while utilizing the properties of reversibly becoming light shielding or opaque or milky white.
With this shortcoming of the above reversible thermosensitive recording material taken into consideration, the inventors of the present invention previously facilitated image erasure (making images transparent) by using a mixture of a higher fatty acid and an aliphatic dicarboxylic acid to broaden the transparentizing temperature width to about 20xc2x0 C. as described in Japanese Laid-Open Patent Applications 2-1363 and 3-2089. This method, however, has a shortcoming that the erasure cannot be sufficiently facilitated when the ambient temperature largely changes or when the heat application time for the erasure is short.
In order to improve such erasability, it is proposed to broaden the transparentizing temperature width by using a mixture of (a) a higher ketone or a fatty acid ester having a lower melting point than those of higher fatty acids, and (b) an aliphatic dicarboxylic acid or a saturated aliphatic bisamide as described in Japanese Laid-Open Patent Applications 4-366682, 5-294062 and 6-255247. This method is capable of broadening the transparentizing temperature width and accordingly capable of improving the erasability. However, due to the use of the higher ketone or fatty acid ester having a lower melting point than those of higher fatty acids, the transparentizing temperature width is situated in a low temperature range, so that this method has a shortcoming that the formed opaque or milky white images formed are erased when the ambient temperature is high.
In order to improve the erasability of the image without lowering the heat resistance thereof, it has been proposed to shift the transparentizing temperature width to a high temperature side by using a mixture of (a) a low-molecular-weight compound having a low melting point and (b) an alicyclic dicarboxylic acid having a melting point of about 200xc2x0 C. which is significantly higher than the melting points of aliphatic dicarboxylic acids (as described in Japanese Laid-Open Patent Applications 5-139053, 6-48024 and 6-48025, or by using a mixture of (a) a low-molecular-weight compound having a low melting point and (bxe2x80x2) a low-molecular compound having a steroid skeleton having a melting point near to 200xc2x0 C. (as described in Japanese Laid-Open Patent Applications 8-20167 and 8-282131). These recording media are capable of improving the erasability while maintaining the heat resistance of the image, but has the shortcomings that the temperature difference between a transparentizing upper-limit temperature and an opaqueness initiation lower-limit temperature is so large that a significantly large amount of energy is required for the formation of milky white images, and that the durability of the media is lowered while in repeated use, with the surface of the recording media scratched, and the opaqueness of the image lowered in the course of the repetition of image printing and erasure.
When a large amount of energy is required for the image formation, a thermal head""s pulse application time is required to be lengthened since there is a limit to a voltage that can be applied to the thermal head from a power source, or the recording speed be lowered. Furthermore, when the amount of energy applied to the thermal head is increased, the life of the thermal head is shortened. Thus, when the amount of energy required for the image formation is increased, the applied energy has adverse effects on an apparatus using the reversible thermosensitive recording medium. In this case, it is considered that the high opaqueness initiation temperature is caused by the use of a low-molecular weight compound having an excessively high melting point.
It is therefore a first object of the present invention to provide a reversible thermosensitive recording medium with an extended transparentizing temperature width, while maintaining the capability of producing images with high heat resistance, and with high repeated use durability, which is capable of producing images with high contrast and erasing the same with high erasability even when the ambient temperature varies.
A second object of the present invention is to provide a method of producing the above reversible thermosensitive recording medium.
A third object of the present invention is to provide an information recording device utilizing the reversible thermosensitive recording medium of the present invention.
A fourth object of the present invention is to provide a method of recording images in any of the reversible thermosensitive recording medium of the present invention and the above-mentioned information recording medium or erasing recorded images therefrom.
The first object of the present invention can be achieved by a reversible thermosensitive recording medium which comprises a reversible thermosensitive recording layer comprising a matrix resin and an organic low-molecular-weight material dispersed in the matrix resin, of which transparency is reversibly changeable depending upon the temperature thereof, with the reversible thermosensitive recording medium having (1) a transparentizing upper-limit temperature of 125xc2x0 C. or more, (2) a temperature difference of 20xc2x0 C. or less between the transparentizing upper-limit temperature and an opaqueness initiation lower-limit temperature, and (3) a transparentizing initiation temperature of less than 95xc2x0 C.
It is preferable that the reversible thermosensitive recording medium further have a transparentizing temperature range of 30xc2x0 C. or more.
It is also preferable that the transparentizing upper-limit temperature of the reversible thermosensitive recording medium be 130xc2x0 C. or more.
It is preferable that in the reversible thermosensitive recording medium, the temperature difference between the transparentizing upper-limit temperature and the opaqueness initiation lower-limit temperature be 15xc2x0 C. or less.
The first object of the present invention can also be achieved by a reversible thermosensitive recording medium which comprises a reversible thermosensitive recording layer formed thereon comprising a matrix resin and an organic low-molecular-weight material dispersed in the matrix resin, of which transparency is reversibly changeable depending upon the temperature thereof, the organic low-molecular-weight material comprising a mixture of at least one straight chain hydrocarbon compound (A) comprising at least one bond selected from the group consisting of amide bond, urea bond and sulfonyl bond, and at least one carboxyl group, and having a melting point of 130xc2x0 C. or more, and at least one straight chain hydrocarbon compound (B) having a melting point which is lower by at least 30xc2x0 C. than the melting point of the straight chain hydrocarbon compound (A).
In the above reversible thermosensitive recording medium, it is preferable that the straight chain hydrocarbon compound (B) have a melting point of less than 100xc2x0 C.
In the above reversible thermosensitive recording medium, it is also preferable that the straight chain hydrocarbon compound (B) have a melting point of 50xc2x0 C. or more.
In the above reversible thermosensitive recording medium, it is preferable that the straight chain hydrocarbon compound (B) and the straight chain hydrocarbon compound (A) be mixed in a mixing ratio by parts by weight of 98:2 to 10:90.
In the above reversible thermosensitive recording medium, it is preferable that as the straight chain hydrocarbon compound (A), a straight chain hydrocarbon compound comprising an amide bond and a carboxyl group be used.
In the above reversible thermosensitive recording medium, it is preferable that as the straight chain hydrocarbon compound (A), a straight chain hydrocarbon compound of general formula (1) be used:
HOOCxe2x80x94(CH2)n-Xxe2x80x94(CH2)m-Yxe2x80x94(CH2)n-COOHxe2x80x83xe2x80x83(1)
wherein 1xe2x89xa6nxe2x89xa626, 1xe2x89xa6mxe2x89xa626, and X and Y each independently represent CONH or NHCO, but do not have an identical structure at the same time.
It is also preferable that in the above reversible thermosensitive recording medium, a straight chain hydrocarbon compound comprising a urea bond and a carboxyl group be used as the straight chain hydrocarbon compound (A).
It is also preferable that in the above reversible thermosensitive recording medium, a straight chain hydrocarbon compound comprising a sulfonyl bond and a carboxyl group be used as the straight chain hydrocarbon compound (A).
In the above reversible thermosensitive recording medium, it is preferable that as the straight chain hydrocarbon compound (A), a straight chain hydrocarbon compound of general formula (2) be used:
CH3xe2x80x94(CH2)n-Zxe2x80x94(CH2)m-COOHxe2x80x83xe2x80x83(2)
wherein 0xe2x89xa6nxe2x89xa625, 1xe2x89xa6mxe2x89xa626, and Z represents NHCONH or SO2.
In the above reversible thermosensitive recording medium, it is preferable that the organic low-molecular-weight material further comprise at least one straight chain hydrocarbon compound (C) in the mixture, having a melting point which is higher by at least 10xc2x0 C. than that of the straight chain hydrocarbon compound (B) and is lower by at least 10xc2x0 C. than that of the straight chain hydrocarbon compound (A).
The second object of the present invention can be achieved by a method of producing a reversible thermosensitive recording medium comprising a support, and a reversible thermosensitive recording layer formed thereon comprising a matrix resin and an organic low-molecular-weight material dispersed in the matrix resin, of which transparency is reversibly changeable depending upon the temperature thereof, comprising the steps of:
coating a dispersion on the support, the dispersion comprising a solvent, the matrix resin and the organic low-molecular-weight material comprising an organic low-molecular-weight compound having a melting point of 130xc2x0 C. or more, which organic low-molecular-weight material is dispersed in the form of a solid in the matrix resin, and
drying the dispersion with application of heat thereto so as to dissolve the organic low-molecular-weight material in the solvent when heat is applied thereto, thereby forming the reversible thermosensitive recording layer on the support.
In the above method, it is preferable that the organic low-molecular-weight material dispersed in the dispersion have a solubility of 0.5% or more in the solvent at a temperature at which the dispersion coated on the support is dried with application of heat thereto.
In the above method, it is also preferable that the organic low-molecular-weight material dispersed in the dispersion have a solubility of less than 0.5% in the solvent at room temperature.
The second object of the present invention can also be achieved by a method of producing a reversible thermosensitive recording medium comprising a support, and a reversible thermosensitive recording layer formed thereon comprising a matrix resin and an organic low-molecular-weight material dispersed in the matrix resin, of which transparency is reversibly changeable depending upon the temperature thereof, comprising the steps of:
coating a dispersion on the support, the dispersion comprising a solvent, the matrix resin and the organic low-molecular-weight material comprising at least one organic low-molecular-weight compound and an organic low-molecular-weight compound having a melting point of 130xc2x0 C. or more, which organic low-molecular-weight materials are dispersed in the form of a solid in the matrix resin, and
drying the dispersion with application of heat thereto at a temperature which is lower than the highest melting point of the melting points of the organic low-molecular-weight materials, and then at a temperature which is not lower than the highest melting point of the melting points of the organic low-molecular-weight materials, thereby forming the reversible thermosensitive recording layer on the support.
The third object of the present invention can be achieved by a card comprising a reversible thermosensitive recording portion which comprises the reversible thermosensitive recording medium of the present invention, and an information memory portion.
In the above card, the information memory portion may comprise at least one element selected from the group consisting of a magnetic recording layer, IC and an optical memory.
The above-mentioned card may further comprise a support and a magnetic recording layer which is provided on one side of the support, and the reversible thermosensitive recording portion is provided on a back side of the support opposite to the magnetic layer.
In the above-mentioned card, the reversible thermosensitive recording portion may further comprise a portion in which an image can be irreversibly printed, or which comprises such irreversibly printed image.
The third object of the present invention can also be achieved by a reversible thermosensitive recording label comprising:
a support,
a reversible thermosensitive recording portion which comprises the reversible thermosensitive recording medium of the present invention, and
an adhesive or tacky layer on a back side of the support opposite to the reversible thermosensitive recording layer of the reversible thermosensitive recording medium.
In the above-mentioned reversible thermosensitive recording label, the reversible thermosensitive recording portion may further comprise a portion in which an image can be irreversibly printed, or which comprises such irreversibly printed image.
The third object of the present invention can also be achieved by a disk cartridge comprising:
a cartridge,
a writable or rewritable disk in which information to be recorded therein is writable or rewritable, which writable or rewritable disk is built in the cartridge, and
a reversible thermosensitive display portion which comprises the reversible thermosensitive recording medium of the present invention or the above-mentioned reversible thermosensitive recording label, which reversible thermosensitive display portion is provided on the surface of the cartridge.
In the above-mentioned disk cartridge, the reversible thermosensitive recording portion may further comprise a portion in which an image can be irreversibly printed, or which comprises such irreversibly printed image.
The third object of the present invention can also be achieved by a disk comprising:
a writable or rewritable disk in which information to be recorded therein is writable or rewritable, and
a reversible thermosensitive display portion which comprises the reversible thermosensitive recording medium of the present invention or the above-mentioned reversible thermosensitive recording label, which reversible thermosensitive display portion is provided on the surface of the writable or rewritable disk.
In the above-mentioned disk, the reversible thermosensitive recording portion may further comprise a portion in which an image can be irreversibly printed, or which comprises such irreversibly printed image.
The third object of the present invention can also be achieved by a tape cassette comprising:
a cassette member,
a writable or rewritable tape member in which information to be recorded therein is writable or rewritable, disposed in the cassette member, and
a reversible thermosensitive display portion which comprises the reversible thermosensitive recording medium of the present invention or the above-mentioned reversible thermosensitive recording label, which reversible thermosensitive display portion is provided on the surface of the tape cassette.
In the above-mentioned tape cassette, the reversible thermosensitive recording portion may further comprise a portion in which an image can be irreversibly printed, or which comprises such irreversibly printed image.
The fourth object of the present invention can be achieved by a method of recording images or erasing recorded images with application of heat to one of recording media selected from the group consisting of the reversible thermosensitive recording medium, the card, the reversible thermosensitive recording label, the disk cartridge, the disk, and the tape cassette mentioned above.
In the above-mentioned method, the application of heat for erasing recorded images may be carried out, using a ceramic heater.
In the above method, it is preferable that the ceramic heater be set at a temperature of 110xc2x0 C. or more for the application of heat for erasing recorded images.
In the above method, the application of heat for recording or erasing recorded images may be carried out, using a thermal head.
When the thermal head is used, the thermal head may apply heat to any of the above-mentioned recording media for erasing recorded images and also for recording images thereon in an overwriting manner.